The present invention relates to a light waveguide sensor for small pulling or pressing forces. More particularly, it relates to such a light waveguide sensor which has a primarily coated light waveguide, a coil wound on the light waveguide, and a casing surrounding the light waveguide and the coil.
Light waveguide sensor of the above mentioned general type are known in the art. One of such light waveguide sensors for pulling forces is disclosed for example in the German document DE-OS 3,526,966. In this reference, a coil wound on the light waveguide is composed of a metal wire, for example a steel wire with a thickness of 0.08 mm or a glass fiber, and then a pulling-resistant casing is applied around them. The casing is composed of a glass fiber reinforced thermoplastic material and is wire-shaped with an outer diameter of approximately 2 mm.
For forming the coils numerous approaches have been taken to the diameter and the pitch length and several coils can be wound around the primarily coated light waveguide in a parallel or cross-lay arrangement. Various materials have been proposed for the casing, for example fiber reinforced synthetic plastic resin (Duroplast), such as polyester resin with unidirectionally oriented glass fibers, and also thermoplasts.
This light waveguide sensor for pulling forces is more sensitive than the older sensor described in the German reference DE-OS 3,305,234 in which the coil of resin-impregnated glass fibers is wound on the light waveguide, or in homogenous synthetic plastic layer with addition of grainy glass or Corundum powder is applied. The utilization of such a sensor is described for monitoring concrete structural works such as a prestressed concrete bridge. The light waveguide sensor is located in a meander-like bedded prestressing wire of the bridge and the measuring ends of the light waveguide are connected with a light-passage testing device (damping measuring device) to allow a continuous mechanical monitoring of the bridge.
Such light waveguide sensors can be easily converted from pulling sensors to pressing sensors.
The German document DE-OS 3,628,083 described a ground plate from a beam with embedded light waveguide pressure sensors, in which the inhomogenous layer between the light waveguide and the casing is formed as a metal wire coil. This ground plate is embedded in buildings or environment for object protection in the ground.
The German Pat. Application P 3,809,957.8 discloses a light waveguide pressure sensor in which the inhomogenous layer between the light waveguide and the casing (protective casing) is formed as the above mentioned grainy synthetic plastic layer. The sensor acts as a signal generator or for release of protective devices at the forces of approximately 1 N. It is used predominantly as contact sensors in the safety technique for clamping, contacting or overriding protection.
In the light waveguide pulling sensor described in the German document DE-OS 3,526,966 only the embodiment with one coil is practically operative. For a sensor without casing (sensor core) composed of a primarily coated multimode lightwave guide with an outer diameter of approximately 175 um and gradiant fiver 50/125 and with a wound steel wire with a diameter of approximately 90 um the following is true:
By applying an axially acting force, this sensor is expanded and the steel wire is constricted into the light waveguide and therefore imparts light damping properties to the light waveguide due to the microbanding effect. The damping increase is linear with the expansion as long as the microbanding effect takes place. In all circumstances this is the case with a expansion of the sensor up to 0.3%. With greater expansions the sensor is rearranged so that finally the steel wire is directly clamped and now the light wave-guide is wound around it. Later on, in this expansion condition the damping of the sensor no longer changes.
A linearization of the damping is achieved by encasing of the sensor with fiber reinforced synthetic plastic material. With suitable selection of the parameters for light waveguide, wire and casing, an expansion sensor with linear damping-expansion ratio up to over 1.5% expansion is produced. Thereby values from 0.2 to 1.5 dB damping per one centimeter expansion are achieved.
The coordination of the parameters of the light waveguide-wire-casing is difficult and complicated to reproduce. This makes the manufacture of such an expansion sensor expensive, time consuming and complicated.